In many applications, a single communications device (e.g., a digital radio handset or a mobile phone) is expected to concurrently receive data over multiple channels. For example, some digital radio handsets include multiple hardware subunits for concurrently demodulating two independent streams of I/O data. Examples of communication standards that can make use of such functionality include the 3G Dual Cell High Speed Downlink Packet Access standard (DC-HSDPA) and the 2G EDGE Evo Down Link Dual Carrier standard (2G DLDC), among others.
In an attempt to concurrently receive data over multiple channels, some conventional communication devices include separate first and second I/O receiver subunits. A first local oscillator (LO) signal generator provides a first pair of LO signals to the first I/O receiver subunit, wherein the first pair of LO signals share a common frequency but are phase-shifted by 90° relative to one another. Similarly, a second LO signal generator provides a second pair of LO signals to the second I/O receiver subunit, wherein the second pair of LO signals also share a common frequency and are also phase-shifted by 90° relative to one another. Although such conventional communication devices may be sufficient in some arenas, cross-talk between the receiver subunits can seriously degrade quality of reception in at least two cases.
First, cross-talk can arise when a first oscillator, which is used to generate the first pair of LO signals for the first receiver subunit, is tuned closely to a second oscillator, which is used to generate the second pair of LO signals for the second receiver subunit. For example, cross-talk can arise when the first and second receivers receive data streams on adjacent 2G channels (e.g., first receiver receives a wanted radio frequency (RF) signal at 2 GHz, and second receiver receives a wanted RF signal at 2 GHz+200 kHz). In this situation, the first and second oscillators deliver nearly the same frequency (e.g., 4 GHz, and 4 GHz+400 KHz, respectively) to first and second divide-by-two frequency dividers, respectively. The close proximity of these oscillation frequencies can generate crosstalk between the receiver subunits making accurate down conversion/demodulation difficult.
Second, cross-talk can also occur when a harmonic frequency used in one receiver subunit is close to the wanted RF frequency (or a harmonic frequency) used in the other receiver subunit. For example consider a receiver where the oscillator supplying the LO signals to the first I/O receiver oscillates at a frequency of 200 MHz (and hence has harmonic frequencies at 400 MHz, 600 MHz, 800 MHz, and so on). If the wanted RF signal for the second I/O receiver has a frequency of 400 MHz, a harmonic frequency from the first I/O receiver can degrade the signal on the second I/O receiver, or vice versa, thereby impeding accurate reception of data.
Therefore, in view of the shortcomings of conventional multi-band receivers within mobile phones and other communication devices, the inventors have devised improved receivers that limit signal degradation due to crosstalk between reception units.